Technical Abstract:
Responses of toll-like receptor 5 (TLR5), tumor necrosis factor (TNF), natural resistance-associated macrophage protein (Nramp), and hepcidin to experimental challenge with Edwardsiella ictaluri in two families of channel catfish were measured in order to understand the mechanisms through which E. ictaluri causes infection. One family demonstrated high susceptibility to infection with E. ictaluri in previous challenges (>90% mortality), while the other family demonstrated very low susceptibility (<20% mortality). Fish were challenged by immersion with virulent E. ictaluri and sampled at 6, 24, 48, and 72 hrs post-challenge. Levels of mRNA expression of TLR5, TNF, Nramp, and hepcidin in liver tissue were measured by quantitative RT-PCR. TNF and TLR5 expression in liver increased significantly at 48 hrs post-challenge in both the high and low susceptibility families (p </=0.0001) and decreased by 72 hrs to levels not significantly different from uninfected controls. There were no significant differences at any time point between high and low susceptibility families, whether infected or uninfected. Expression rates in infected high and low susceptibility families differed significantly from uninfected siblings only at 48 hrs post-challenge. Nramp expression in liver increased significantly at 24 hrs post-challenge in both the high and low susceptibility families (p </= 0.0001), remained unchanged at 48 hrs, and decreased by 72 hrs to levels not significantly different from uninfected controls. There were no significant differences between infected high and low susceptibility families at any time point. Expression rates in infected low susceptibility families differed significantly from uninfected siblings only at 24 hrs post-challenge (p </= 0.0001). There were no significant differences in expression between infected high susceptibility fish and their uninfected siblings; however, Nramp expression was significantly higher in uninfected low susceptibility fish than in the high susceptibility fish (p = 0.004) at 24 hrs. Hepcidin expression in liver remained unchanged in both the high and low susceptibility families at 6, 24 and 48 hrs, increasing significantly at 72 hrs (p = 0.0001) only in the low susceptibility family. There were no differences in expression levels in uninfected high or low susceptibility controls. In this study, only hepcidin show differential expression patterns in high and low susceptibility families. This data will be combined with expression studies with spleen and trunk kidney tissue from these same fish, to determine if these expression patterns hold true for other tissues as well. These results may provide insight into how E. ictaluri causes infection in catfish with different susceptibility phenotypes.